The present invention relates to a cellulose based fiber of high water retentivity for use as a water-retentive material in an absorbent member absorbing body fluids in sanitary napkin, disposable diaper, incontinence pad and the like, and a method of manufacturing the same and a water-retentive sheet prepared from the fiber.
Absorbent members are arranged at areas receiving body fluids such as urine and blood of menstruation, in sanitary goods such as disposable diaper and sanitary napkin. The absorbent members have a structure such that pulp or a super absorbent polymer (referred to as xe2x80x9cwater-retentive materialxe2x80x9d hereinafter) is interposed between a liquid pervious sheet such as nonwoven fabric and a liquid impervious sheet such as polyolefin. In recent years, it has been demanded to prepare these sanitary goods as compact type and slim type. Thus, it is required to improve performance and shape stability of the water-retentive material in the absorbent members.
Absorbent materials of powdered polymer and absorbent materials of fibrous polymer have been known conventionally as a water-retentive material, and as described in xe2x80x9cJournal of Industrial Materialsxe2x80x9d, Vol.42, No.4, p.18, generally, absorbent materials of powdered polymer are used.
As the absorbent members of powdered polymer, it has been known synthetic polymers such as polyacrylate based compounds and polyvinyl based compounds as well as natural polymers such as cyanomethyl cellulose and carboxymethyl cellulose.
As the absorbent members of fibrous polymer, the following fibers have been known; a fiber produced by a process of mixing sodium salt of carboxymethyl cellulose with viscose prior to spinning, as described in Japanese Patent Laid-open (kokai) No. 56-9418; a fiber produced by a process of carboxymethylating regenerated cellulose fiber, as described in Japanese Patent Publication (kokoku) No. 60-2707; and a fiber of a bilayer structure, produced by hydrolyzing an acrylonitrile fiber, thereby forming a polyacrylate based absorbent layer on the outer surface, as described in Japanese Patent Laid-open (kokai) No.55-132754.
For using such water-retentive materials in absorbent members of sanitary goods such as disposable diaper and sanitary napkin, the materials are required to have high absorbency. Furthermore, it is also required that the water-retentive materials have a property such that water once absorbed into the materials should not be released from the materials even under pressure, namely so-called high water retentivity.
For using the fibrous water-retentive materials as the water-retentive materials in absorbent members, the fibrous water-retentive materials are required to have a fiber strength of about 0.8 g/denier (g/d) at their dry state, from the respect of handling of the fibrous water-retentive materials at manufacturing stages.
However, such powdered water-retentive materials come off easily from the absorbent members. The water-retentive materials turn into a gel state with high fluidity in a water-absorbed state, disadvantageously, so such materials are poor in terms of shape stability.
For using the powdered water-retentive materials as a water-retentive material in absorbent members of disposable diaper and the like, the water-retentive materials turn into a gel state within the disposable diaper, when the water-retentive materials absorb urine. Following the motion of a wearer with such disposable diaper thereon, the gel makes a sift with the resultant uneven distribution of the gel in the absorbent member. Additionally, the gel is sticky. Therefore, the wearer feels unpleasant touch and poor feeling during use.
Because the viscose and carboxymethyl cellulose in the fibrous water-retentive material produced by mixing the sodium salt of carboxymethyl cellulose with viscose are both cellulose base, these are highly compatible with each other. Therefore, the water-retentive material has good characteristics as fiber. However, the water retentivity is not sufficient.
In the fibrous water-retentive material produced by carboxymethylating rayon, because the fiber has water absorbency as a whole, the fiber of itself turns into a gel state when the material absorbs water. Accordingly the material are poor in terms of shape stability. Disadvantageously, the fiber strength is low in a dry state.
The fibrous water-retentive material of such bilayer structure, produced by forming a polyacrylate based absorbent layer on the outer surface of an acrylonitrile based fiber, is disadvantageous in that the process of producing the water-retentive material is complex.
In accordance with the present invention, the aforementioned problems are to be solved. The present invention provide a fiber of high water retentivity which is safe for use as absorbent members of sanitary goods such as disposable diaper and sanitary napkin, which also has a high water retentivity, greater shape stability because the fiber can retain the fiber shape even in a water-absorbed state, and a fiber strength sufficient enough for handling at its dried state, as well as an absorbent member wherein the fiber of high water retentivity is used.
The present invention relates to a cellulose based fiber of high water retentivity comprising a cellulose fiber which contains uniformly a non-cellulose based material of high absorbency.
In the cellulose based fiber of high water retentivity of the present invention, a cellulose fiber and an material of high absorbency are sufficiently mixed together to an extent such that the fiber and the material which can absorb water cannot be discriminated from each other, so that the material of high absorbency is uniformly dispersed in the cellulose fiber. Both the cellulose fiber and the material of high absorbency have high water absorbency. and high water retentivity. Accordingly, the cellulose based fiber of high water retentivity uniformly containing the two components is more excellent in terms of absorbency and water retentivity than conventional fibers singly composed of cellulose or the super absorbent polymers (SAP). Even at mechanic processing stages such as yarn splitting stage or at a water-absorbed state, the material of high absorbency hardly comes off from cellulose based fiber of high water retentivity. When the fiber absorbs water, the material of high absorbency exposed to the outer surface of the cellulose based fiber of high water retentivity may eventually come off. The other hand, there is an advantage such that water can be efficiently absorbed by the material of high absorbency on the outer surface.
Additionally, the cellulose based fiber of high water retentivity of the present invention includes a complex fiber wherein a component of cellulose fiber which contains uniformly a non-cellulose based material of high absorbency and a single component of cellulose are attached to each other in a side by side type.
Furthermore, the fiber of the present invention includes a complex fiber wherein a core is formed from a component of cellulose fiber which contains uniformly a non-cellulose based material of high absorbency and the core is enveloped with a sheath prepared from a single component of cellulose.
In the said complex fiber of side by side type, a component containing a material of high absorbency uniformly dispersed in cellulose fiber is attached to the single component of cellulose, wherein the component containing the material of high absorbency has water absorbency and water retentivity while the single component of cellulose retains the mechanical properties as a fiber. Therefore, the resulting fiber has high water absorbency and high water retentivity, together with higher fiber strength and greater shape stability.
The said complex fiber of sheath-core type wherein the core prepared from the component of the material of high absorbency uniformly dispersed in cellulose fiber is attached to the sheath prepared from the single component of cellulose, has a structure such that the component containing the material of high absorbency (core) is covered with the single component of cellulose (sheath). Even at a water-absorbed state or even at any stage of the fiber production, therefore, the material of high absorbency does not come off from the fiber. By preparing the sheath component as a thin coating film, then, water absorbency can be retained.
The complex fibers of the side by side type and the sheath-core type have higher absorbency and water retentivity and also have higher dry strength of the fiber produced by uniformly dispersing the material of high absorbency in the cellulose fiber than the fiber prepared from the single component, even when the content of the material of high absorbency in the cellulose fiber in the complex fiber is equal to the content of the material of high absorbency in the cellulose fiber in the fiber composed of a single component.
In accordance with the present invention, the cellulose fiber primarily means viscose-rayon fiber. However, other hydrophilic cellulose fibers may be used satisfactorily.
In accordance with the present invention, furthermore, the material of high absorbency primarily means polyacrylate salt. The polyacrylate salt is commercially available, generally and readily, as polyacrylate based absorbents or polyacrylate based super absorbent polymers. (Journal of Industrial Materials, Vol.42, No.4, p.26.) The polyacrylate based absorbents or polyacrylate based super absorbent polymers are absorbent polymers primarily comprising slightly cross-linked polyacrylate salt, polyacrylate salt grafted onto starch or polyacrylate backbone, and these may be used singly or in combination with two or more thereof. Furthermore, an isobutylene-maleic anhydride copolymer may be used. As the material of high absorbency, additionally, use may satisfactorily be made of super absorbent polymers based on polyvinyl alcohol or polyoxyethylene.
The absorbency of the cellulose based fiber of high water retentivity of the present invention is 700% or more. The term absorbency herein means a value represented by the following formula 1;
xe2x80x83V(%)={(Bxe2x88x92A)/A}xc3x97100xe2x80x83xe2x80x83(Formula 1)
wherein A is the weight in gram of the fiber prior to water absorption; and B is the weight in gram of the fiber after water absorption and draining.
The water retentivity of the cellulose based fiber of high water-retentive is 200% or more. The term xe2x80x9cwater retentivityxe2x80x9d herein means a value represented by the following formula 2;
W(%)={(Dxe2x88x92-C)/C}xc3x97100xe2x80x83xe2x80x83(Formula 2)
wherein C is the weight in gram of the fiber prior to water absorption; and D is the weight in gram of the fiber after water absorption and draining and subsequent centrifuge for dehydration.
As described above, the cellulose based fiber of high water retentivity has higher water absorbency and water retentivity. In both a dry state and a water-absorbed state, the cellulose based fiber of high water retentivity can retain the fiber shape. When the fiber is enveloped in a paper sheet to form an absorbent member for use in disposable diaper and sanitary napkin, the fiber does not make any shift in the disposable diaper and the sanitary napkin. Thus, disposable diapers and sanitary napkins with high water absorbency and water retentivity can be provided while a wearer will not feel any unpleasant touch therewith.
Alternatively the cellulose based fiber of high water retentivity can be prepared as sheet form or can be knitted into other fiber webs or nonwoven fabric. Then, an absorbent member may satisfactorily be prepared from those. The resulting absorbent member thus formed has higher water absorbency and water retentivity even if it is so slim in its thickness. Therefore, when the absorbent member is used in disposable diaper and sanitary napkin, the resulting disposable diaper and sanitary napkin can be prepared as slim type.
Furthermore because the polymer forming fiber in the cellulose based fiber of high water retentivity of the present invention is not a synthetic polymer substance such as polyacrylonitrile but cellulose, it has such properties to be readily degradable and is further rapidly degradable in soil.
At a process of manufacturing the cellulose based fiber of high water retentivity into a sheet form or at a process of mixing the fiber into other fiber webs or nonwoven fabric, preferably, the dry strength of the fiber is 0.8 g/denier (g/d) or more and the fineness thereof is 5 denier or more to 15 denier or less, for easy handling of the fiber. The unit of dry strength, namely xe2x80x9cg/dxe2x80x9d, means the tensile strength of a fiber corresponding to one denier. When the fineness is above 15 denier, furthermore, the water absorbency is reduced. Therefore, the fineness is preferably 15 denier or less.
Additionally, more preferably, other super absorbent polymers and pulp may be mixed with the fiber. A plurality of the sheets, nonwoven fabric or fiber web, containing the cellulose based fiber of high water retentivity of the present invention, are laminated together or held between paper sheets from both the upper face and lower face, followed by adhesion. After adhesion, then, the resulting sheet is molded into a given shape to form an absorbent member. Otherwise, the sheets, nonwoven fabric or fiber web, containing the cellulose based fiber of high water retentivity of the present invention, may be molded into a given shape, prior to adhesion. Or the cellulose based fiber of high water retentivity is mixed with a hot-melt type fiber, followed by thermal processing to prepare a sheet of a given shape. Because the cellulose based fiber of high water retentivity in this sheet is securely bonded to each other through the hot-melt type fiber, the shape is hardly broken. At the process of bonding the sheets, furthermore, the sheets can be thermally bonded to each other. At this thermally-bonding process, the sheets can be uniformly bonded as a whole. Preferably, the water-retentive sheet contains the cellulose based fiber of high water retentivity at 10% by weight or more to 80% by weight or less, while the sheet contains the hot-melt type fiber at 20% by weight or more to 80% or less.
The basis weight of the sheet containing the fiber of high water retentivity is preferably 10 g/m2 or more to 500 g/m2 or less.
The method of manufacturing the cellulose based fiber of high water retentivity in accordance with the present invention comprises spinning, elongation and refining a stock solution for spinning as a raw material which is a homogeneous mixture of a non-cellulose based material of high absorbency with cellulose fiber.
So as to produce a complex fiber of side by side type or sheath-core type, the stock solution of a homogenous mixture of a non-cellulose based material of high water absorbency with the cellulose based component is mixed with a stock solution component singly composed of cellulose fiber by means of a nozzle, which is then spun, elongated and refined.
For using viscose-rayon fiber as the cellulose fiber and polyacrylate salt as the non-cellulose based material of high absorbency in the fiber of high water retentivity of the present invention, routine viscose for viscose-rayon fiber is used for the stock solution. Term xe2x80x9croutine viscose for viscose-rayon fiberxe2x80x9d primarily means viscose for general viscose rayon, at a cellulose concentration of 7% by weight or more to 10% by weight or less and an alkali concentration of 5% by weight or more to 6% by weight or less and with a Hottenroth number of 8 to 12. As the alkali in this viscose, primarily, use is made of sodium hydroxide. Otherwise, any viscose with a modified composition of the individual components in the viscose may satisfactorily be used. Otherwise, viscose for strong rayon, viscose for polynosic, or viscose for HWM may also be used.
For using polyacrylate salt as the non-cellulose based material of high absorbency, the polyacrylate salt is satisfactorily mixed with the stock solution of viscose. Then, the amount of the polyacrylate salt to be mixed should be at 10% by weight or more to 200% by weight or less to the total weight of the cellulose fiber in the viscose. If the amount thereof to be mixed is less than 10% by weight, the water retentivity is not sufficient enough. If the amount thereof is above 200% by weight, alternatively, the polyacrylate salt is present excessively in the stock solution of viscose, which causes poor stringiness in a regeneration bath during spinning, disadvantageously for smooth spinning.
At the process of manufacturing the cellulose based fiber of high water retentivity, treatment with an alkaline solution is preferably carried out after refining. The alkaline solution to be used for the alkali treatment is preferably an aqueous sodium carbonate solution or an aqueous sodium bicarbonate solution. Through such alkali treatment, the absorbency and water retentivity of the fiber can be enhanced.